It's nice to be backed up by stats but all of the following:
-90s had a much deeper CC field, especially the early-mid 90s.
-Thiem isn't nearly as good as advertised. The guy who doesn't have a single big CC title by the age of 26/27 is not another Guga/if not for Nadal, regardless of his impressive consistency at the FO.
is self evident IMO.
I've moved your post around somewhat cuz I forgot to add one more point which is apropos here: forget 30% of return games won, it's actually the 40%
threshold that almost guarantees a Coupe des Mousquetaires in the current year or at some other point in the player's career. For illustration here are all the RG champs since '91, except Rafa (haha), who in their prime have cracked or flirted with (defined as an unrounded 39%+, relevant years in brackets) the 40% mark:
Courier - 
Bruguera - , 93, 94
Chang - 94, 
Muster* - 95, 
Costa, A. - 96
Ferrero - 01
Agassi - 03
Gaudio - 05
(Muster also won 39.5% in '91 but wasn't a serious contender that year. And for completeness' sake Lendl also posted 39.9% in '93 but flamed out early at RG in the 1st round.)
Failing to make this list are Kafelnikov, Kuerten, Moya, Federer, Wawrinka and Djokovic. (FYI Novak did manage 42.0% in '05 but was a nonfactor on clay till at least '06. His 2nd-highest RGW average after '05 was 38.7% in '10.) That's only 8 players (again apart from Rafa) and just a dozen CC campaigns out of who knows how many since the ATP began tracking this stat in '91*.
Even when you consider the creeping upticks in % of service games won it really is telling that only one of these guys was born in the '80s or beyond... and just barely at that (JCF's DOB is 2/12/80)! And even when you look at the % of overall games won Novak is the only one not named Rafael Nadal who cracked the all-important 60% mark in the '10s ('11, '14, '15, in addition to '08 and '09). In fact for the latest non-Djokodal instance you need to go all the way back to '05 when Fed and Gaudio won 60.7% (again his DC stats are excluded) and 59.7% respectively. That's not a whole lot of depth, especially when the numbers also suggest that the strongest RG challenger even at his absolute peak would be barely competitive with Rafa's weaker versions.
So yeah I definitely think the '90s had a deeper field, even without factoring in the lack of surface customization (as opposed to specialization, mind you) these days. And that doesn't bode well for Thiem's chances at RG when he struggles to win even 30% of his return games on dirt.
*Just for fun here are the former FO runner-ups and SFists who managed to post or flirted with 40% of RGW:
Ferrer - 02, 07, , 
Coria - 03, 04, 05, 
Nalbandian - , 05, 09
Murray - 11
It goes without saying none of 'em were serious contenders in at least half of these years (Ferru in '02 and less so in '07, Coria in '06, Nalby in pretty much all of his three)... and in the other half all of these guys combined managed to make the top 10 in % of service games won only once! (Perhaps the most egregious case was Andy in '11, when he made the FO SF despite winning a pathetic 72% of service games on clay.) In fact that was the only time Coria ever cracked 80% (82.0% in '03) in the serving department, and though Ferru and Muzz have done it more often - five times for David (81.3%, 82.1%, 87.1%, [79.6%] and 80.4% in '10, '11, '12, '14 and '15 respectively) and twice for Andy (85.8 in '15 and 83.6% in '16) - it wasn't quite enough to make up for the drop in their RGW% (37.5%, 33.7%, 35.4% and 37.9% in '10, '12, '14 and '15 respectively for Ferru and 34.6 in '15 and 32.7% in '16 for Muzz) except David's 39.7% in '11. Oh yeah, and you'd have to be blind to think Ferru and even Coria were gonna overwhelm anyone with heavy groundies a la Nadal, Bruguera, Courier or even Muster, as the diminutive Argie found out at '03 RG when Martin friggin' Verkerk blew him off the court with one of the finest offensive displays on dirt (as Moose would tell you).
-Sampras was a tough player to beat in a one-off match on clay. It's a combination of several factors that stopped him from being a legit FO contender in his era (not that Ferrer ever was either, mind you).
While browsing YT to see if the '95 Pete-Gaudenzi DC rubber had finally become available (though perhaps I should stop looking - like I said it was a dead rubber) I came across this earlier one in full:
Blur Pete's mug/bod in this vid and I doubt anyone would be able to call him out for his supposedly hopeless sliding that everyone is so sure about. And check out the patented running FH to close out the 1st-set TB (point starts at 1:16:50) which did plenty of damage throughout the match. Call me crazy but I don't think the Sampras-was-a-mug-on-clay canard stands up to scrutiny!
(BTW I won't pretend to have seen the whole thing or even be familiar with the guy's game but Furlan seems to have had a sweet DTL BH himself. One of those dirtballing floaters that used to be a dime a dozen.)
-Fed (and I'd argue Novak as well despite his impressive stats on clay) wouldn't be making 5 FO finals in the 90s, on the other hand Pete would have a FO final or two in this era.
Novak I actually think might equal his final appearances, though obviously not all in the '90s. The guy may not be a born dirtballer like Rafa or Bruguera but he's solid as a rock.
And I do think Fed would eke out a FO in any era though he'd need some luck (like almost anyone else). I also think Pete would make at least one or two FO finals now, though that depends on how committed to clay he'd be (which would be easier today, but still difficult enough to make him think twice).
Now regarding Ferrer being a true dirtballer, that I'm not sure. I can't even tell if he's better on clay than on HC. He has had more success on clay but there's much more depth on HC.
The first slam breakthrough he made was 2007 USO when he reached a SF there. He also had similar results at AO compared to FO.
Murray and Hewitt had great return stats for example but clay is their worst surface. Chang was also IMO better on HC than on clay despite winning FO.
I've long maintained that Ferru is actually more dangerous on HC myself, but that's really thx to his 1st-rate return
which tends to do more damage on faster courts. Of course that can only take you so far. I still remember catching his action during one of his Wimby runs (think it was vs. Delpo in '12, but not sure), and after seeing almost nothing but half-moonballs from the baseline I knew he had absolutely no chance at topping any of the legit GCers in the SF if he ever got past the QF.
Having said that I think Ferru's superior CC stats to either Rusty or Muzz (both on serve and return) despite being a lesser player in more ways than one (believe he's even shorter than Hewitt) is pretty convincing evidence that the guy was a legit dirtballer. He's a Spaniard after all and his comfortable sliding allows him to play from well beyond the base/sidelines and change direction without planting the foot. That allows him and his dirtballing peers to maximize their potential and play their otherwise superior opponents tough despite having no big weapon on paper (guess I'll put in an obligatory shout-out to the '05 Rome final between Rafa and Coria
Yup, players just hit way harder now on clay because of poly and the resultant spin. Even back then, Berasatagui could do that on the forehand side and Kuerten was using Luxilon and was already vaporising balls at RG.
They say the clay mixture has changed at some of the tourneys but I've yet to see any evidence of that. So yeah, clay looks faster now cuz 1) poly and 2) same HC tennis everywhere. The operative word being "looks."